A device of this type for controlling an operating characteristic of an internal combustion engine is known from U.S. Pat. No. 4,635,607. There, a fuel-metering system is presented with an actuator for influencing the supply of air to the internal combustion engine, with means which detect a driving condition critical to safety by checking the drive signal of the air-influencing actuator with respect to limit values. Furthermore, this means detects a fluctuation of the idling speed by checking the speed as a function of time and the means also, in the presence of a driving condition critical to safety, modifies the condition for fuel cut-off in such a way that, increasing linearly as a function of speed, this condition is still valid for openings of the throttle flap of up to 5.degree..
Moreover, it is known from U.S. Pat. No. 4,311,123 to increase the fuel quantity according to a selectable time function by opening the idling contact after deceleration has ended. This allows a smooth transition to the normal driving mode.
Furthermore, it is known from U.S. Pat. No. 4,305,359 to detect a defect of the actuator for guidance control by checking the actuator position and accelerator-pedal position. A defect is assumed when the accelerator pedal is in the idling position and the actuator, especially the throttle flap, is outside its idling position. When such a defect is present, the fuel supply is cut off in order to reduce the speed. When the driver moves the accelerator pedal out of its idling position again, the cut-off of the fuel supply is cancelled in order to avoid driving conditions critical to safety. The speed of the internal combustion engine can thereby be increased again.
However, it is a disadvantage that, when the throttle is opened, the sudden torque jump can give rise to an undesirable vehicle acceleration, with the result that the driver is possibly no longer in control of the situation.
It is the object of the invention to improve the driving performance during the transition from the idling mode to the driving mode in a device for controlling an operating parameter of an internal combustion engine of the type mentioned above and to prevent driving conditions critical to safety.
Furthermore, a method for controlling the fuel-metering system of an internal combustion engine in the overrun mode is known from U.S. Pat. No. 4,549,519. There, the actual speed is compared with a predetermined speed threshold value which is reduced from a high initial value to a low final value according to a time function. If the actual speed is above this speed threshold, the fuel supply is cut off. Conversely, when the speed is lower than this speed threshold value, the fuel supply resumes. The transition from the overrun mode to the normal operating state is not described herein.
In the following, the term "idling state" covers all the operating states in which the accelerator pedal or throttle flap are in their idling position, especially the overrun mode.